Melanotic tumors in tumor-W mutant larvae of Drosophila melanogaster are aggregates of posterior fat body cells encapsulated by hemocytes. Examination of the abnormal region in tumorous larvae with the scanning electron microscope has revealed that blood cell aggregation around the adipose cells follows disruption of the basement membrane surrounding the tissue. The proposed research will be an investigation into the nature of this surface covering and its role as a recognition site in cell-mediated reactions of Drosophila. This study will utilize a series of melanotic tumor strains and will involve a detailed examination of cell-cell interactions utilizing the scanning electron microscope. In addition, various tissues and cell types, including cells growing in culture, will be implanted in Drosophila larvae to test the effectiveness of each cell type in eliciting hemocyte response. Analysis of the observations will correlate cell surface features and properties of tissue membranes (inductive system) with hemocyte response (reactive system). Biochemical components of isolated basement membrane, and metabolic pathways leading to basement membrane formation will be studied.